Method for the purification of prostaglandins

ABSTRACT

The present invention provides a method for the purification of a prostaglandin by supercritical fluid chromatography, said method comprising the use of a stationary phase and a mobile phase comprising carbon dioxide, provided that when the stationary phase is unmodified silica gel, the prostaglandin is not luprostiol. The invention also provides prostaglandins obtainable by the method.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/953,236, filed Jul. 29, 2013, which is a continuation of U.S. patentapplication Ser. No. 12/820,610, filed Jun. 22, 2010, which is now U.S.Pat. No. 8,519,178, issued Aug. 27, 2013, and claims priority to U.S.Provisional Application No. 61/219,166, filed Jun. 22, 2009, thedisclosures of which are incorporated herein by reference in theirentireties for all purposes.

FIELD OF THE INVENTION

The present invention provides a method for the purification ofprostaglandins. In particular, the present invention provides a methodfor the purification of prostaglandins by supercritical fluidchromatography (SFC).

BACKGROUND OF THE RELATED ART

Prostaglandins are active pharmaceutical ingredients (APIs) andisomerically and chemically pure prostaglandins are required forformulation into drug products. However, the purification ofprostaglandins is challenging due to the similar chemical properties ofmany prostaglandin isomers, as well as their related impurities.

G. H. Brunner et al. (Supercritical Fluids, 653-668, E. Kiran and J. M.H. Levelt Sengers (eds.), Kluwer Academic Publishers, 1994) describesthe preparative supercritical fluid chromatography (SFC) separation ofReprodin isomers (i.e. luprostiol isomers). However, the isomers arepoorly separated and the purity of the heart cut fraction is only 80%.This method therefore cannot be used to obtain a commercial product asthe purity does not meet ICH quality. The authors acknowledge that SFCis disadvantageous in comparison to liquid chromatography.

BRIEF SUMMARY OF THE INVENTION

The present inventors, however, have overcome the problems associatedwith the prior art procedure to provide an alternative process for thepurification of prostaglandins using SFC.

Accordingly, the present invention provides a method for thepurification of a prostaglandin by supercritical fluid chromatography,said method comprising the use of a stationary phase and a mobile phasecomprising carbon dioxide, provided that when the stationary phase isunmodified silica gel, the prostaglandin is not luprostiol. For example,the invention includes a method for purifying a crude prostaglandin,comprising injecting the crude prostaglandin onto a column comprising astationary phase, eluting the crude prostaglandin through the columnusing a mobile phase comprising supercritical carbon dioxide, andcollecting a fraction comprising a purified prostaglandin.

DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a chromatogram of a SFC purification injection ofcrude latanoprost, obtained as described in Example 1.

FIG. 2 sets out the structures of latanoprost isomers.

FIG. 3 illustrates a chromatogram of a SFC purification injection ofcrude latanoprost, obtained as described in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

By “purification”, it is meant the method produces a prostaglandin whichis chemically and/or isomerically pure. A chemically pure compound isone where the compound is essentially free from related compounds,chemical starting materials, chemical intermediates and chemicaldegradants. An isomerically pure compound is one where the compound isessentially free from known related compounds of the same chemicalmolecular formula that are different in chemical structure.

In one embodiment, the purified prostaglandin is at least about 99%chemically pure, preferably at least about 99.5% chemically pure andmore preferably at least about 99.8% chemically pure.

In another embodiment, the purified prostaglandin is at least 99%isomerically pure, preferably at least about 99.5% isomerically pure andmore preferably at least about 99.9% isomerically pure.

The method of the present invention may be utilised to purify aprostaglandin on an analytical or preparative scale. By “analytical”, wemean a scale of providing at least about 0.1 mg of purifiedprostaglandin, preferably about 1 mg of purified prostaglandin, in areasonable timeframe, i.e. less than a day. By “preparative”, we mean ascale of providing at least about 1 mg of purified prostaglandin,preferably about 0.1 g of purified prostaglandin and most preferablyabout 1 g of purified prostaglandin, in a reasonable timeframe, i.e.less than a day.

Preferably, the prostaglandin is a compound of formula (I), a compoundof formula (II), a compound of formula (III) or a compound of formula(IV):

wherein,V is C or O;W is

X is CONR₁₂R₁₃ or CO₂R₁₂;Y is

Z is C₁₋₂₀-alkyl, C₂₋₂₀-alkynyl, —O—(C₆₋₂₀-aryl) or—(C₁₋₂₀-alkyl)-(C₆₋₂₀-aryl), wherein the aryl group is optionallysubstituted with one to three substituents selected from the groupconsisting of C₁₋₂₀-alkyl, halo and C(halo)₃;R₁ and R₂ are independently H or OH, or R₁ and R₂ together form

R₃ and R₄ are independently H or OH;R₅ and R₆ are independently H or OH, or R₅ and R₆ together form

R₇ and R₈ are independently H, OH, halo or C₁₋₂₀-alkyl;R₉ and R₁₀ are independently H or C₁₋₂₀-alkyl, wherein the alkyl groupis optionally substituted with one or more substituents selected fromCONR₁₂R₁₃, CO₂R₁₂ or CO₂ ⁻M⁺;R₁₁ is C₁₋₂₀-alkyl or —O—(C₁₋₂₀-alkyl), wherein the alkyl group isoptionally substituted with one or more substituents selected fromCONR₁₂R₁₃, CO₂R₁₂ or CO₂ ⁻M⁺;R₁₂ and R₁₃ are independently selected from the group consisting of Hand C₁₋₂₀ alkyl;M⁺ is a counter cation; andHalo is fluorine, chlorine, bromine or iodine. When V═C, it isunderstood that the carbon atom bears two hydrogen atoms, one or both ofwhich may be substituted with a substituent group which may beindependently selected from halo and C₁₋₂₀-alkyl, for example.

“Alkyl” refers to linear, branched or cyclic saturated hydrocarbonstructures having, unless otherwise indicated, 1 to 20 carbon atoms,more preferably 1 to 15 carbon atoms and most preferably 1 to 10 carbonatoms. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl,cyclopropyl, n-butyl, i-butyl, t-butyl, n-pentyl, n-hexyl andcyclohexyl. When an alkyl group having a specific number of carbonsatoms is named, it is intended that all geometric isomers of that alkylgroup are encompassed. For example, “butyl” includes n-butyl, i-butyl,t-butyl and cyclobutyl.

“Alkynyl” refers to linear or branched hydrocarbon structures having atleast one

group and, unless otherwise indicated, 2 to 20 carbon atoms, morepreferably 2 to 15 carbon atoms and most preferably 2 to 10 carbonatoms. Examples of alkynyl groups are ethynyl, propynyl, n-butynyl,isobutynyl and hexynyl. When an alkynyl group having a specific numberof carbon atoms is named, it is intended that all geometric isomers ofthat alkynyl group are encompassed. For example “butynyl” includesn-butynyl and isobutynyl.

“Aryl” refers to an aromatic hydrocarbon structure having, unlessotherwise indicated, 6 to 20 carbon atoms, more preferably 6 to 15carbon atoms and most preferably 6 to 10 carbon atoms. Examples of arylgroups are phenyl and naphthyl.

The term “halo” whether alone or as part of another group refers to ahalogen, for example, a fluorine, chlorine, bromine or iodine atom.

M⁺ is a counter cation of —CO₂ ⁻ i.e. —CO₂ ⁻M⁺ is a carboxylic acid saltand preferably a pharmaceutically acceptable carboxylic acid salt.Preferably M⁺ is a metal ion e.g. an alkali metal ion, such as K⁺ orNa⁺.

Preferably, W is

X is preferably CONH(C₁₋₁₀-alkyl), CO₂(C₁₋₁₀-alkyl) or CO₂H. Morepreferably, X is selected from the group consisting of CONHEt, CO₂Me,CO₂ ^(i)Pr and CO₂H.

Preferably, Z is C₁₋₁₀-alkyl, C₂₋₁₀-alkynyl, —O—(C₆₋₁₀-aryl) or—(C₁₋₁₀-alkyl)-(C₆₋₁₀-aryl), wherein the aryl group is optionallysubstituted with one to three substituents selected from the groupconsisting of Cl and —CF₃. More preferably, Z is —(CH₂)₅CH₃, —(CH₂)₃CH₃,—CH₂-Ph,

R₁ and R₂ may independently be H or OH. Preferably, when one of R₁ andR₂ is H, the other of R₁ and R₂ is OH. More preferably, when one of R₁and R₂ is H and the other of R₁ and R₂ is OH, R₁, R₂ and the carbon atomto which they are attached have the following stereochemistry:

R₃ and R₄ are independently H or OH. Preferably, when one of R₃ and R₄is H, the other of R₃ and R₄ is OH. More preferably, R₃, R₄ and thecarbon atom to which they are attached have the followingstereochemistry:

R₅ and R₆ may independently be H or OH. In one embodiment, when one ofR₅ and R₆ is H, the other of R₅ and R₆ is OH. More preferably, when oneof R₅ and R₆ is H and the other of R₅ and R₆ is OH, R₅, R₆ and thecarbon atom to which they are attached have the followingstereochemistry:

When R₅ and R₆ together form

and one of R₃ and R₄ is OH and the other of R₃ and R₄ is H, it ispossible for a tautomer to be produced i.e. a hemiacetal. It isenvisaged that such tautomers are encompassed with the scope of thepresent invention. A tautomeric equilibrium is exemplified by theprostaglandin lubiprostone:

In another embodiment, R₅ and R₆ are both H.

Preferably, R₇ and R₈ are independently selected from the groupconsisting of H, OH, F or CH₃. In one embodiment, R₇ and R₈ are both H.In another embodiment, R₇ and R₈ are both F. In yet another embodiment,one of R₇ and R₈ is CH₃ and the other of R₇ and R₈ is H or OH.

Preferably, R₉ and R₁₀ are independently H or C₁₋₁₀-alkyl, wherein thealkyl group is optionally substituted with one or more substituentsselected from CO₂H or CO₂ ⁻M⁺. More preferably, one of R₉ and R₁₀ is Hand the other of R₉ and R₁₀ is —(CH₂)₃CO₂H or —(CH₂)₃CO₂ ⁻Na⁺.

Preferably, R₁₁ is C₁₋₁₀-alkyl or —O—(C₁₋₁₀-alkyl), wherein the alkylgroup is optionally substituted with one or more substituents selectedfrom CO₂H or CO₂ ⁻M⁺. More preferably, R₁₁ is —(CH₂)₃CO₂H or—O—CH₂—CO₂H.

In a preferred embodiment, the prostaglandin is selected from the groupconsisting of:

Preferably, the prostaglandin is latanoprost.

Alternatively, if the stationary phase is other than unmodified silicagel, the prostaglandin can be luprostiol:

In one embodiment, the stationary phase is a chiral stationary phase.Preferably, the chiral stationary phase is a derivatised amylose orcellulose polymer or other polysaccharide which is coated or immobilizedon silica. More preferably, the chiral stationary phase is selected fromthe group consisting of Chiralcel OD-H, ChiralPak AS-H, ChiralPak IC,ChiralPak AD-H, Chiralcel OJ-H and Chiralcel OK (products available fromChiral Technologies Inc. and Daicel Chemical Industries, Ltd.). Morepreferably, the chiral stationary phase is ChiralPak AD-H. For example,the amylase or cellulose polymer may be derivatised with one or morecarbamate groups, especially aryl-containing carbamate groups such as3,5-di methylphenylcarbamate, (S)-alpha-methylbenzylcarbamate,4-chlorophenyl carbamate, 4-methylphenylcarbamate, phenyl carbamate,3-chloro-4-methylphenylcarbamate, 5-chloro-2-methylphenylcarbamate orthe like, and/or one or more ester groups, such as acetate, benzoate(e.g., 4-methyl benzoate), cinnamate, or the like.

In another embodiment, the stationary phase is a non-chiral stationaryphase. Preferably, the non-chiral stationary phase is selected from thegroup consisting of Princeton Diol, 4-ethyl pyridine, 2-ethyl pyridineand pyridine urea.

Preferably, the mobile phase further comprises at least one modifier.The modifier can be any suitable liquid solvent. A suitable modifier maybe selected from the group consisting of at least one alcohol,acetonitrile, ethyl acetate, methylene chloride and a combinationthereof. Preferably, the at least one alcohol is selected from the groupconsisting of methanol, ethanol, propanol, isopropanol and a combinationthereof. It is desirable that the modifier is compatible with thestationary phase. For example, ethyl acetate and methylene chloridecannot be used with a ChiralPak AD column as they will destroy thecolumn.

Carbon dioxide is easily removed and so, the purified prostaglandin canbe provided as a solution with the modifier as a solvent. It maytherefore be desirable to select a modifier in which the prostaglandinis soluble.

Suitably, the at least one modifier is present in a quantity from (i.e.,of at least) about 1% v/v or about 1% w/w to the supercritical carbondioxide. More preferably, the at least one modifier is present in aquantity from (i.e., of at least) about 5% v/v or about 5% w/w to thesupercritical carbon dioxide. The ratio of modifier to carbon dioxidecan be varied during the chromatographic process.

Suitable chromatographic apparatus is well known to the skilled person.It is preferred to use an apparatus that is suitable for SupercriticalFluid Chromatography such as the Thar Investigator SFC or NovasepSupersep 20/30 SFC. The crude feed containing the crude prostaglandin isperiodically injected into the apparatus wherein the mobile phase flowsthrough the stationary phase which is located in a column. Afterdetection at the column outlet, the purified fractions of the feed aredirected to different traps. The carbon dioxide is removed from thepurified fractions and is preferably recycled. Detection at the columnoutlet can be conducted by measuring UV absorption at an appropriatewavelength.

On an analytical scale, the column diameter is suitably from about 2 mmto about 7 mm, preferably about 4.6 mm. The column length is suitablyfrom about 5 cm to about 50 cm, preferably about 25 cm.

On a preparative scale, the column diameter is suitably from about 10 mmto about 200 mm, preferably about 21 mm. The column length is suitablyfrom about 5 cm to about 50 cm, preferably about 25 cm.

The process is suitably carried out at temperatures from about 5° C. toabout 45° C., preferably about 25° C. to about 35° C., and at elevatedpressures e.g. from about 80 bar to 300 bar, preferably about 100 bar to150 bar. Typical flow rates depend upon the diameter of the column andmay vary from e.g. 1 mL/min to about 5 kg/min.

In another aspect, the present invention provides a prostaglandinobtainable by a method as described above.

The following Examples are illustrative but not limiting of theinvention.

Example 1 Analytical Scale

SFC Preparatory Parameters:

-   -   Column: Chiral Technologies Chiralpak AD-H, 4.6×250 mm    -   Column Temperature: 35° C.    -   System Back Pressure: 150 Bar    -   Eluent: Carbon Dioxide (CO₂) with an alcoholic modifier, Ethanol    -   Total Flow Rate: 3 mL/min    -   Gradient Co-Solvent Profile: Initial conditions=5% Ethanol: 95%        CO₂, linearly increase to 45% Ethanol: 55% CO₂ in 15 minutes,        maintain 45% Ethanol: 55% CO₂ for 2 minutes, ramp back to        initial conditions and equilibrate for 3 minutes prior to next        injection    -   Detection: UV at 210 nm    -   Sample Preparation: Prepare a 0.1 g/mL solution in Ethanol and        thoroughly mix the feed solution to completely dissolve the        Latanoprost    -   Sample Loading: 5 mg (50 μL of 0.1 g/mL crude solution in        Ethanol)    -   Latanoprost Collection: Collect the Latanoprost peak (heart cut)        from approximately 2% above the baseline from the detected        Latanoprost peak front (at ˜9 minutes in retention time) to the        Latanoprost detected peak tail (at ˜10 minutes in retention        time).    -   The chromatogram of a SFC purification injection of Latanoprost        Crude is provided in FIG. 1.    -   Obtained Results using Ethanol as the Co-Solvent and AD-H        Stationary Phase:

Chromatographic Purity - Reported as Measured % Peak Area LatanoprostIsomeric Content* Iso- C11- C15R- C15S- C15S- Sample Chemical meric betatrans cis trans Crude Feed 95.87% 90.02% 1.11% 3.88% 3.04% 0.05% (0.01g/mL) Heart Cut 99.95% 99.92% — — 0.08% — (5 mg Loading) *The structuresof the latanoprost isomers are set out in FIG. 2.

Example 2 Preparative Scale

SFC Preparatory Parameters:

-   -   Column: Chiral Technologies Chiralpak AD-H, 21×250 mm    -   Column Temperature: 25° C.    -   System Back Pressure: 100 Bar    -   Eluent: Carbon Dioxide (CO₂) with an alcoholic Co-solvent        modifier (specifically Ethanol: Methanol (4:1))    -   Total Flow Rate: 50 g/min    -   Co-Solvent Profile: Initial conditions=15% Co-solvent: 85% CO₂,        for 360 seconds, Step to 40% Co-solvent: 60% CO₂ at 361 seconds        until 720 seconds, step back to initial conditions at 721        seconds until 900 seconds    -   Detection: UV at 220 nm    -   Crude Solution Preparation: Prepare a 0.3 g/mL solution in        Ethanol and thoroughly mix the feed solution to completely        dissolve the Latanoprost    -   Sample Loading: ˜0.6 mL crude solution in Ethanol    -   Latanoprost Collection: Collect the Latanoprost peak (heart cut)        from approximately 30 seconds after peak start the baseline from        the detected Latanoprost peak front (at ˜410 seconds in        retention time) to approximately 30 seconds prior to the peak        end (at ˜510 seconds in retention time).    -   The chromatogram of a SFC purification injection of Latanoprost        Crude is provided in FIG. 3.    -   Obtained Results using Ethanol as the Co-Solvent and AD-H        Stationary Phase of the collected fractions:

Chromatographic Purity - Reported as Measured % Peak Area LatanoprostIsomeric Content^(§) Iso- C11- C15R- C15S- Sample Chemical meric betatrans C15S-cis trans Crude 96.51% 93.07% 0.70% 2.79% 2.15% 0.06% FeedPurified 99.81%* 99.98% — — 0.02% — Heart Cut *No impurities measuredabove 0.04% by percent peak area. ^(§)The structures of the latanoprostisomers are set out in FIG. 2.

-   -   No isomers were detected after 11 months storage at freezer        temperature when the purified Latanoprost was analyzed with a        typical HPLC method for isomer content determination to the        following detection levels:

Isomer Detection Limit C11-beta 0.03% C15R-Trans 0.03% C15S-Trans 0.05%C15S-Cis 0.03% Latanoprost 0.03%

What is claimed:
 1. A latanoprost composition comprising C11-beta isomerof latanoprost in an amount greater than 0% and less than 0.03%, andcontaining less than 0.03% C15R-Trans isomer of latanoprost, less than0.05% C15S-Trans isomer of latanoprost, and less than 0.03% C15S-Cisisomer of latanoprost, after 11 months storage at freezer temperature.2. A latanoprost composition comprising a C11-beta isomer of latanoprostin an amount greater than 0% and less than 0.03%, after 11 monthsstorage at freezer temperature.